There's an excellent reason why the United States' top infectious disease expert isn't gathering with family outside his home for Thanksgiving this year.

Infected people without symptoms, who might feel well, continue to unwittingly spread the coronavirus on a large scale.

In fact, the CDC recently released an updated scientific brief noting the coronavirus — which has already killed well over a quarter of a million Americanssince February — is spread mostly by people who are asymptomatic or presymptomatic (meaning they don't yet have symptoms). That's a big reason why the public health agency recommends everyone wear masks in public, and also suggests only celebrating Thanksgiving with people in your household.

The coronavirus is an especially insidious parasite because the hosts (us) often don't know we're carrying a pathogen that's actively multiplying in our respiratory tract. There can be no telltale symptoms.

"This means, at least half of new infections come from people likely unaware they are infectious to others," the CDC noted in a separate report about transmission and masks, updated Nov. 16. Specifically, some 24 percent of those who transmit the virus will never show symptoms, and 35 percent are pre-symptomatic. (Though the total number of infected people who are asymptomatic is between 40 to 45 percent, yet not everyone is out transmitting the disease.)

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Some might argue that people can "safely" visit family members on Thanksgiving by promptly getting tested, and testing negative, before strolling into someone's house to enjoy a long feast. This is wrong. Generally, it takes days for the virus to potentially show detectable levels in a diagnostic test (like a PCR test, which tests for genetic material). Sure, you could test negative a day after flying somewhere. But that's meaningless.

"For example, it’s not a good idea to fly into Boston on a crowded flight, get a COVID-19 diagnostic test within a day or two of arrival, and then, based on a negative result, visit your elderly grandparents," according to MIT Medical, a medical clinic in Cambridge, Massachusetts. "The only thing that negative test can tell you is that, at that particular moment in time, your sample did not show viral levels high enough to be reliably measured."

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There is undoubtedly "light at the end of the tunnel" when it comes to fighting COVID-19, specifically from promising vaccine candidates. But the first vaccinations, if given emergency FDA approval, won't begin until December, and many Americans likely won't receive vaccinations for many months, perhaps sometime in spring, according to Dr. Anthonly Fauci, the head of infectious diseases at the National Institutes of Health.

That's why masks remain vital, and gathering indoors with others during a nationwide third surge remains foolish. Reported cases are increasing, significantly, nearly everywhere. At least we know wearing masks in public spaces, like when we're grocery shopping, vastly reduces the spread of the coronavirus. Here's a salient example: In Kansas, counties that implemented a mask mandate since July 3 saw a six percent decrease in COVID-19 cases. In sharp contrast, counties with no mandate saw a 100 percent increasein cases, according to the CDC.

SEE ALSO:How to celebrate the holidays alone, from Thanksgiving to Christmas

But at the Thanksgiving table, the masks will come off. The virus is powerless without us, relying on hosts to spread, which we do by talking, breathing, sneezing, and beyond. We give the virus power.

"This is a virus that we know is very happy to take advantage of people being careless," Dr. Vince Silenzio, an M.D. and professor at the Rutgers School of Public Health, told Mashable.

Read more about COVID-19

• What Dr. Fauci's doing for Thanksgiving

• 'It's like a f*cking horror movie:' Healthcare workers vent about COVID-19 conspiracy theorists

• COVID-19 patients are developing ‘brain fog.’ But what does that mean?

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